By Kirk Miller May is considered one of the best times to buy a mattress because many manufacturers put out new products in June — ergo, they need to get rid of some old stock. Tack on a long weekend with Memorial Day (ideal for shopping deals) and you’ve got pretty much every major mattress brand offering hundreds of dollars off. For most of these sleep brands, the deals actually start now — and end before the weekend actually beings. Others, however, go through the weekend or even into June. And some sales (not listed here) don’t start until later in the week, so definitely check back on the…